1. Field of the Invention
The present invention relates to a position measuring device for measuring the relative positions of two objects. The two objects to be measured are machine components of a processing or working machine or a measuring machine for tools in which the graduation of a scale connected to the first object is scanned by a scanning unit connected to the second object. The scale is arranged on a scale carrier so as to be slightly displaceable in measuring direction. The scale carrier is connected with both ends thereof to fastening elements which, in turn, are rigidly connected to the first object.
2. Description of the Prior Art
Position measuring devices of the above-described type are used particularly in processing machines for measuring the position of a tool relative to a workpiece to be processed as well as in machines for measuring coordinates for determining the position and/or dimensions of test objects.
Devices are already known in position measuring apparatuses for the compensation of errors, particularly errors in graduation of a scale, of machine errors of machine components and/or of temperature-related errors.
German patent No. 27 12 421 discloses an enclosed longitudinal measuring device in which a scale is longitudinally movably arranged on a scale carrier and is connected through fastening elements at both ends to a machine component to be measured. The fastening elements, in turn, are rigidly fastened to the machine component. While one end of the scale is connected directly to one of the fastening elements, the other end of the scale is connected through a spring-biased expanding device to the other fastening element in order to correct graduation errors and/or machine errors. The longitudinal measuring device according to German Offenlegungsschrift No. 29 11 047 has a compressing device in addition to an expanding device.
German patent No. 25 18 745 discloses an enclosed longitudinal measuring device whose housing is fastened at both ends thereof through an angle member each to a machine component in an articulated manner. A scale is longitudinally movably arranged in the housing and, for correcting graduation errors and/or machine errors, the scale is connected at both ends thereof to the angle members through a spring-biased expanding device or a spring-biased compressing device.
German patent No. 31 06 701 is directed to a device in a longitudinal measuring apparatus for machine tools in which, for the compensation of length changes of machine components due to temperature changes, an expansion rod of a material having a high coefficient of thermal expansion is connected with one end thereof to a machine component and with the other end thereof to a scale, wherein the thermal expansion of the expansion rod is equal to that of the machine component, so that the thermal expansion of the machine component is determined by the measuring device.
German Offenlegungsschrift No. 28 53 771 describes a longitudinal measuring device in which a housing for receiving a scale and a scanning device is connected at both ends thereof through fastening elements to a machine component to be measured. While one end of the housing is directly connected to a first fastening element, a length compensating element for the thermal compensation is arranged between the other end of the housing and the second fastening element. Thus, the latter end of the housing is supported with a translatory degree of freedom in longitudinal direction of the housing.
German Offenlegungsschrift No. 32 43 966 discloses a longitudinal measuring device in which a scale is arranged in the middle thereof on a rigid reference point and is otherwise arranged on a scale carrier so as to be slightly movable in measuring direction when temperature changes occur. The scale carrier is connected on both ends thereof through a length compensating element each to fastening elements which are rigidly fastened on a machine component to be measured. The middle of the scale carrier is fastened through an additional fastening element rigidly to the machine component, so that the scale maintains its fixed reference point relative to the machine component when temperature changes occur.
U.S. Pat. No. 3,816,002 describes a longitudinal measuring device with a housing for receiving a scale and a scanning device. One end of the housing is rigidly connected to a machine component and the other end of the housing is longitudinally movably connected to the machine component for the compensation of length changes due to temperature changes. The scale is fixed at one end thereof within the housing and is connected with the other end thereof through a tensioning device to the housing. Due to the spring-biased tensioning device, the scale is not influenced by temperature-related length changes of the housing.
The compensation of temperature-related errors is necessary because generally a constant temperature does not exist in spaces which house processing machines on which the above-described position-measuring devices are used. In addition, the temperatures of the individual processing machines continuously rise during the duration of workpiece processing. Since, for cost reasons, rarely the machine part to be measured, the scale as well as the scale carrier will be of materials having the same thermal expansion coefficient .alpha., the unavoidable temperature changes of the machine component, the scale and scale carrier result in different thermal length changes, so that measuring inaccuracies may occur during the processing of a workpiece. Such inaccuracies are not acceptable today because high demands are made of the accuracy of processing or workpieces. The material most frequently used for the machine component is gray cast iron (.alpha.(O)=10.10.sup.-6 K.sup.-1), while aluminum (.alpha.(G)=23.10.sup.-6 K.sup.-1) is used for the scale carrier and glass (.alpha.(M)=8.10.sup.-6 K.sup.-1) is used for the scale. In addition, the workpieces most frequently processed are of steel (.alpha.(W)=11.10.sup.-6 K.sup.-1).
German Offenlegungsschrift No. 34 19 527 describes a position measuring device used on a processing machine in which a bending-resistant scale of glass is arranged in a housing as a scale carrier so as to be movable in measuring direction. The housing is connected with one end thereof directly to a first fastening element and with the other end thereof through two calibrating screws to a second fastening element. The scale has a greater graduation length than the required intended graduation length and the two fastening elements act at the ends of the scale. The scale is compressed by means of the two calibrating screws through the two fastening elements at a normal temperature of 20.degree. C. until the intended graduation length is reached. Since the machine component to be measured generally will not have this normal temperature when the position measuring device is mounted, the two calibrating screws are provided with temperature scales which make it possible to adjust the graduation length of the scale to the temperature of the machine component. After the two fastening elements have been rigidly fastened to the machine component, the two calibrating screws are released. Because of the compression of the scale, the scale and the machine component are subjected to the same thermal length changes when the temperature changes. This position measuring device is of relatively complicated construction and is not able to compensate thermally caused processing inaccuracies if the machine component and the workpiece to be processed are not composed of materials having the same thermal expansion coefficient .alpha..
It is, therefore, the primary object of the present invention to provide a position measuring device of the above-described type which can be mounted on a machine component without having to take the temperature conditions into consideration. In addition, thermally caused processing inaccuracies of the workpiece to be processed are to be avoided even when the scale and the workpiece have different expansion coefficients.